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Corneal Oxygen Deficiency
Oxygen In Relations To Corneal Health
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About COD
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What is COD?
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= Corneal Oxygen Deficiency
Chronic levels of hypoxia.
Also called “cumulative corneal stress”, “corneal
exhaustion” or “corneal fatigue syndrome”.
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How quickly can O
COD
happen?
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A 210% decrease in oxygen to the cornea causes
hypoxic changes in as little as 300 seconds (5
minutes!).
The effects of COD can add up over hours, days &
years!
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Who Gets COD?
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COD
has been documented to occur in soft contact2
lens wearers.
500,000 of soft contact lens wearers may suffer from
corneal problems due to lack of oxygen and do not
realize it.
Early signs may be subtle and difficult to detect with
standard clinical instrumentation.
Wearing low Dk/t contact lenses for long days, 7
days a week and napping in lenses puts a wearer at
risk of COD.
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About Oxygen & Ocular
Health
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How
does
the
cornea
get
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if it doesn’t
have
blood
vessels?
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Unlike most human tissue, the corneal is avascular,2
rendering it transparent, and it does not rely on the
blood supply for nutrients.
The oxygen required for corneal metabolism is
derived principally from the atmosphere, diffused
across a thin tear layer to the anterior corneal
surface.
Traditional contact lens forms a barrier, reducing the
amount of oxygen available to the cornea.
Reduced oxygen supply can result in changes to the
epithelium, stroma and endothelium.
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Why
does
the
cornea
to
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breathe?
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The epithelium and endothelium control the amount of water in the
cornea. They need oxygen to pump water in and out.
If they don’t have oxygen, the cornea SWELLS. It gets THICKER. This
is called Corneal Oedema.
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The open eye
conjunctiva
eyelid
eyelashes
OXYGEN
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cornea
eyelashes
eyelid
conjunctiva
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The closed eye
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3.2% corneal swelling every night.
This is normal.
eyelid
conjunctiva
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OXYGEN
eyelid
conjunctiva
Oxygen comes from the palpebral conjunctiva and the aqueous
humour, some may come from the atmosphere is the lids are not fully
closed.
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O2 Daily wear contact lenses
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Lenses need to be oxygen permeable
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eyelid
conjunctiva
OXYGEN
contact lens
eyelid
conjunctiva
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O2Napping in contact lenses
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Lenses need to be highly oxygen permeable
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eyelid
conjunctiva
OXYGEN
eyelid
conjunctiva
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O2eyes
What
happens
if
the
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don’t
get
enough
oxygen?
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If 2oxygen is blocked to the cornea with a low Dk/t
(i.e. low oxygen transmission) contact lens, over
time:
The eyes become red and bloodshot.
The endothelial layer of your cornea is damaged. If one cell
dies, the neighbours enlarge to fill the gap – polymegethism
(many shapes).
Blood vessels grow from the limbus (edge of eye) into the
cornea –neovascularisation (new vessels).
The eye becomes more vulnerable to infection.
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What does COD look like?
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COD Signs O2
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Eyes become more red/bloodshot
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Corneal endothelium is damaged – polymegethism
Blood vessels grow into the cornea –
neovascularisation
Refractive error shift (0.50D or more) (myopic shift)
Corneal edema
Corneal staining/epithelial integrity compromised
Increased bacterial binding, the eye becomes more
vulnerable to infection
*occurs to varying degree
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O2 Eyes suffering from COD
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Chronically red eyes
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30-35%
of patients in
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low Dk hydrogels had
some degree of
limbal redness
35-40% had
conjunctival redness
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O2 What does COD feel like?
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The contact lens wearers might not feel COD.
But they may notice they can‘t wear their lenses for
long days.
They may even suffer some end-of-day symptoms of
discomfort and dryness.
Two large studies have shown that symptoms of
dryness are related to discontinuation of lens wear.
(Doughty 1997, Begley 2001).
COD FACTS18
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Oxygen Transmissibility
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What is Dk? O2
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Diffusion coefficient (D)
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Solubility coefficient (k)
Dk = Oxygen Permeability
In order for oxygen to pass through a contact lens material, the molecules must
first dissolve into the material and then travel through it.
Dk is independent of lens design or thickness
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High Dk and low Dk
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Dk is Oxygen Permeability
Two lenses of the same
thickness:
High Dk – more oxygen
Low Dk – less oxygen
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What is Dk/t?
O2=oxygen transmissibility
Dk/t
Dk = oxygen permeability
T = lens thickness
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Dk/t takes the material and
the design into account.
It is a better measure of
how much oxygen is
getting to the eye.
This lets us make
comparisons between
contact lens types/brands.
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The
effects
of
lens
power
on
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transmissibility
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• Product guides quote the Dk/t for
a -3.00DS lens.
• Any increase in lens thickness
reduces Dk/t (oxygen
transmission), even if the material
(Dk) is the same.
• So, because higher power lenses
(both plus and minus) are thicker
than lower power lenses, they will
have lower Dk/t. Less oxygen will
get through.
• So for a given design and
material, higher power lenses
often have lower oxygen
transmissibility.
Higher Rx
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Lower Dk/t
-10.00 DS
-3.00 DS
Lower Rx
Higher Dk/t
+3.00 DS
+10.00 DS
Higher Rx
Lower Dk/t
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How much oxygen is
enough?
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The
oxygen
requirements
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avoid
signs
of
corneal
oxygen
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O2
deficiency
are high O2
Daily Wear – short days/part time wear – Dk/t 24
Extended or continuous wear – Dk/t 125
Not all soft contact lenses on the market meet these
criteria
What about long days and occasional napping in
lenses?
The Dk/t for today’s daily wear schedules should be
much higher than 24
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Harvitt and Bonano 1999 . Holden Mertz 1987
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Why isn’t DK/t 24 always
enough?
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There
is a high variability in oxygen metabolism and
there is no simple test to predict oxygen
requirements for individual lens wearers.
Wearers experience increased oxygen needs
resulting from environmental conditions (altitude,
dryness, etc.).
87% patients wear their lenses for very long days
and 84% of patients currently nap in their lenses3.
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CIBA Vision. Data on file.
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Why
isn’t
DK/t
24
O2
always
enough?
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There
2 is a high variability in
oxygen metabolism!
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Average
Highest
Lowest
% 10
oedema 8
6
4
2
0
Baseline
Sleep
Holden, Mertz & McNally 1983
Waking
30 Min
1 Hr Post 6 Hr Post
12 Hr
Post
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Why
isn’t
DK/t
24
always
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enough?
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How do you know in advance how
much oxygen you’ll need?
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O2Why
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isn’t DK/t 24
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always
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Patients experience
increased oxygen
needs resulting from
environmental
conditions (altitude,
dryness, etc.)
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Why
isn’t
DK/t
24
always
O2
enough?
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Long wearing
days
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More contact lens users
nap in their lenses:84% !
CIBA Vision U&A Study 2003
with 2700 consumer in Europe
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History
of contact lens
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1508: Leonardo da Vinci introduced the concept of contact lens: placing the
eyes on a container full of water, which changes the angle of corneal refraction
1880’s: contact lens made of glasses
1936: contact lens made of plastic (PMMA)
1962: soft lens available in the market. In the next 10 years till now, soft lens is
widely accepted by most users
Advantages: High water content, softer lens, more comfortable, oxygen
permeable
1971: Launch of PMMA lens with Silicon content, which is the materials used
for modern hard oxygen permeable lens (RGP)
Advantages: Good optical performance, effective in rectifying
astigmatism/presbyopia, control worsening of short-sightedness, high
oxygen permeability, easy to clean
1998: Launch of soft lens with Silicon content, i.e. Silicone Hydrogel; it
combines the benefits of both soft lens and hard oxygen permeable lens (RGP)
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The 2nd generation of high
permeable contact lenses
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Revolutionary silicone
hydrogel
technology
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A 2new standard for all contact lenses.
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Silicone hydrogel material and a biocompatible
surface.
Permanent chemically bonded surface treatment that
enhances wettability and biocompatibility.
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Dk/t and H2O
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Dk/t
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H20
175
160
138
140
120
110
100
86
80
55
60
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55
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24
20
0
Conventional Lenses
Silicone Hydrogels
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2005-1-0005
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Hydrogel lens vs. silicone
O2 hydrogels
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Ordinary hydrogel lens
after 6 hours
Same eye silicone hydrogel
after 30 nights’ wear
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Patient
benefits
of
silicone
O2
hydrogel
:high
Dk/t
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Promotes long-term corneal health:
Reduces Epithelial permeability (Lin, Polse, et al)1
Decreases
Reduces Acidosis (Lin, Polse, et al)1
Reduces Corneal oedema (Fonn, et al)3
Reduces Bacterial adherence (Ren Cavanaugh)4
Reduces Endothelial polymegathism (Tighe, et al)5
Reduces Limbal injection (Papas)6
1-7
Data on file
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Patient
benefits
of
silicone
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hydrogel
:
safer
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Safer: Higher Dk/t than traditional hydrogels
Promotes long-term corneal health.
Low water content:
Suitable for dry eyes
Decreased dehydration
Brighter, whiter, healthy-looking eyes
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Patient
benefits
of
silicone
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hydrogel
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Ideal for soft contact lens wearers who:
Want breathable lenses for healthy daily wear.
Want outstanding all-day comfort.
May exhibit signs or symptoms of corneal oxygen
deficiency.
May have stopped wearing lenses due to lens
discomfort or corneal oxygen deficiency.
Want to wear lenses for a full day – however long
that may be.
Want value in a high-performance lens.
Want whiter, brighter, healthy-looking eyes.
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Conclusion
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People
in Hong Kong are health conscious, but the findings
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show that most respondents tackle their COD problems
inappropriately and not dealing with the root cause of the
issue.
COD symptoms such as eye redness and bloodshot, can be
improved by wearing high oxygen permeable contact lenses,
but polymegethism and neovascularisation are irreversible,
some patients even may not be able to wear contact lenses
thereafter. Although there are not many cases, we should be
aware of this problem.
Since wearing low oxygen permeable contact lenses will cause
long term defeat to our corneal health, the newly introduced
contact lenses made with silicon hydrogel will improve our
corneal health effectively and suit the modern living style.
Contact lens wearers should visit their optometrists every 6
months for their eye check on contact lens use and oxygen
permeability.
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